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Thread: IMSAI 8080 need wiring verification on PSU section

  1. #31

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    I think I missed this - you bought an IMSAI? Wow.

    Might be visiting you soon for a nose! If you'll have me that is...

  2. #32
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    Quote Originally Posted by JonB View Post
    I think I missed this - you bought an IMSAI? Wow.

    Might be visiting you soon for a nose! If you'll have me that is...
    Of course you would be welcome Jon.

  3. #33

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    Quote Originally Posted by zippysticks View Post
    Agree Dwight - it doesn't look right at all. On the front cap both positive and negative wires have been added to the positive terminal.
    The two large caps on the 8V line are rated at 95000uF 15V for the smaller one and the larger one at 80000uF 15V

    I also think the smaller cap at the rear for the 16V line has its connections the wrong way around too.

    The wiring needs a tidy up so I can trace it through.

    I'm pretty sure the rectifier board is connected up properly - al least from a colour code perspective.
    I believe the two smaller capacitors are wired correctly. One is for the positive 18V and the other is for the negative 18V. If you look carefully at the picture you can see that the rubber buttons for the vents are on the right and left, with the black ground wires on the side away from the card cage. Either they are both wrong or both right. They are what I saw in other pictures of this IMSAI power supply arrangement so I believe they are correctly wired. It is just the black wires on the larger capacitor that look wrong.
    Dwight

  4. #34
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    So I have managed to tidy the PSU wiring up a little.

    Only change I think I needed to make is the move of the black (negative) wire from the smaller of the two 8V smoothing capacitors to the positive terminal.
    As far as I can tell everything now looks correct.

    1. Mains power goes direct to the rectifier board, live through a fuse and off to the front panel switch. It is also fed off to the transformer and mains fan.
    2. Transformer output is fed back to the rectifier board, rectified: 8V through a pair of heatsink cooled power diodes and +-16V through what appears to be a lower powered diode rectification stage.
    3. +- 16V goes off to their respective smaller smoothing caps before reaching the S100 motherboard
    4. 8V goes off to the parallel pair of large smoothing caps at 80000uF and 95000uF before reaching the S100 motherboard

    All sounds reasonable yes ?


    IMG_20190620_160911704.jpg

    I must admit I don't like all that mains power at the front panel switch so might introduce a higher power rated switch at the rear of the unit.
    Last edited by zippysticks; June 20th, 2019 at 08:00 AM.

  5. #35
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    Quote Originally Posted by Dwight Elvey View Post
    I believe the two smaller capacitors are wired correctly. One is for the positive 18V and the other is for the negative 18V. If you look carefully at the picture you can see that the rubber buttons for the vents are on the right and left, with the black ground wires on the side away from the card cage. Either they are both wrong or both right. They are what I saw in other pictures of this IMSAI power supply arrangement so I believe they are correctly wired. It is just the black wires on the larger capacitor that look wrong.
    Dwight
    Hi Dwight,

    Hi just double checked the smaller caps and I believe the black wire is incorrectly connected to the positive terminal of the smaller rear capacitor.
    Note that the two smaller capacitors are rotated 180 degrees from each other.
    Last edited by zippysticks; June 20th, 2019 at 08:29 AM.

  6. #36
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    And now after double checking the wiring of the smaller caps, I have rotated the smaller rear capacitor (top right hand side of picture) 180 degrees and swapped polarity of the wiring so yellow is connected to the positive terminal.

    IMG_20190620_173435306.jpg

    I have tested the caps with a meter and they all seem to perform with a low voltage charge.
    I have also buzzed through all the rectifier board diodes and check for continuity to the caps and transformer.
    Fuse is intact.

  7. #37

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    Quote Originally Posted by zippysticks View Post
    And now after double checking the wiring of the smaller caps, I have rotated the smaller rear capacitor (top right hand side of picture) 180 degrees and swapped polarity of the wiring so yellow is connected to the positive terminal.

    IMG_20190620_173435306.jpg

    I have tested the caps with a meter and they all seem to perform with a low voltage charge.
    I have also buzzed through all the rectifier board diodes and check for continuity to the caps and transformer.
    Fuse is intact.
    Stop!!!!!!!!!!!!!!

    It was right to start with on the two smaller capacitors.
    The black wire is the ground lead. The two supplies are for a negative and a positive rail one needs the black lead on the positive terminal because it is a negative rail. The other needs the black lead on the negative terminal because it is a positive supply.
    Please do not list to the others, The two +8V supplies look correct.
    Dwight
    Last edited by Dwight Elvey; June 20th, 2019 at 09:37 AM.

  8. #38

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    Quote Originally Posted by zippysticks View Post
    Hi Dwight,

    Hi just double checked the smaller caps and I believe the black wire is incorrectly connected to the positive terminal of the smaller rear capacitor.
    Note that the two smaller capacitors are rotated 180 degrees from each other.
    They are suppose to be rotated. The one on the right Is a POSITIVE rail It is suppose to have the black lead on the NEGATIVE side of the capacitor!!!!!!!!!!!!!!!!!!!!!!!
    The other is a NEGATIVE rail is is suppose to have the black lead on the POSITIVE side of the capacitor!!!!!!!!!!!!!!!!!!!!!

    Dwight

  9. #39
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    Yes I wondered after I swapped it and took a look at those archive photos and saw the caps rotated there too.
    Last edited by zippysticks; June 20th, 2019 at 09:58 AM.

  10. #40

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    Quote Originally Posted by zippysticks View Post
    Yes I wondered after I swapped it and took a look at those archive photos and saw the caps rotated there too.
    Look at photo
    http://www.vcfed.org/forum/showthrea...t=power+supply
    post #21

    Even though the two capacitors have the positives post on the same side, the black ground wire crosses over. Who ever put your machine together rotated the capacitor so that the black ground lead goes straight instead of crossing over.
    Dwight

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